[clang] [clang][analyzer doc] Add basic description of checker 'core.CallAndMessage' (NFC). (PR #177179)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat approved this pull request.

LGTM, thanks for the updates!

As this commit only adds documentation, I think you should feel free to merge it. (The RST markup seems to be correct, but for the sake of safety run `ninja docs-clang-html` before merging the patch.)

https://github.com/llvm/llvm-project/pull/177179